1. Field of the Invention
This invention relates generally to the field of phase jitter cancellation in high speed data modems and more particularly to an arrangement for reduction of phase jitter having a sinusoidal component.
2. Background of the Invention
As the speed of operation of data modems continues to increase, the problem of dealing with phase jitter in the transmitted data signal becomes increasingly bothersome. The effect of phase jitter is a rotation of the transmitted symbol without changing the magnitude of the symbol. This can cause the received symbol to differ from the transmitted symbol causing transmission errors. The receiver's immunity to jitter decreases as the number of symbols in the constellation increases. Although the symbol constellations used in such high speed modems have evolved to relatively optimal designs, phase jitter can still create errors which a high speed data modem must correct. It is therefore important to provide a mechanism for canceling as much of the phase jitter in a transmitted signal as possible.
Some types of phase jitter are non-sinusoidal and may be traceable to various randomly occurring electrical signals which may be familiar to those skilled in the art. Another type of phase jitter results when the transmitted data is corrupted by a sinusoidally varying source. This results in sinusoidal phase jitter which is the type of jitter the present invention addresses. The most frequent cause for sinusoidally varying phase jitter is related to ripple in the power supplies used by the telephone company in it's carrier communications equipment. Other sources of phase jitter also produce a slowly varying or approximately sinusoidal component of phase jitter which can be effectively canceled by utilizing the present invention. In the U.S. these sinusoidal components are generally integral multiples of 60 hertz. This is also true in Canada. In some parts of Europe, such as Great Britain, the fundamental frequency of the power line is 50 hertz and in such instances the sinusoidal component of phase jitter is frequently an integral multiple of 50 hertz. According to published data sinusoidal components may exists in transmitted data signals at frequencies between approximately 20 hertz and approximately 300 hertz.
In attempting to correct for such sinusoidally varying phase jitter, numerous problems are encountered. For example, it is possible to lock onto such sinusoidally varying components of phase jitter and produce a signal which cancels the phase jitter. But, in so doing the signal which is being locked to effectively disappears or becomes so reduced in amplitude as to become very difficult to maintain lock on. Additionally, in order to effectively compensate for such phase jitter a relatively wide range of frequencies must be locked upon. This becomes a problem in and of itself but is complicated by the fact that it is desirable to use a very narrow noise bandwidth phase locked loop so that a high degree of noise immunity is maintained. Therefore, use of a simple wideband phase-locked loop or Costas loop may accomplish significant phase jitter reduction, but at the expense of signal to noise ratio. It is also desirable in such a phase jitter canceling system to provide a mechanism for detecting when the phase jitter is small enough that the compensation is not needed. In this instance, the phase jitter correction circuitry can be disabled further enhancing the modem's noise performance.
These and other problems have been effectively addressed in the present invention and such features have been provided in order to enhance the operation of the phase jitter canceling circuitry so that data errors due to sinusoidally varying phase jitter are rare.